Among hydrometallugical work done during the last two decades have been efforts to separate metals, such as gold and silver from gangue material or host rock by leaching with a solvent, and specifically with cyanide solutions. Much of the early work was done by the Reno Metallurgy Research Center of the US Bureau of Mines, Reno, Nev..sup.(1). The present invention relates to an improved method of extracting metals from ores by the chemical leaching principle and specifically to non-agitation leaching methods, such as heap leaching by percolation. FNT .sup.(1) Inter alia see H. J. Heinen, D. G. Peterson & R. E. Lindstrom "Silver Extraction from Marginal Resources", paper delivered at the annual meeting of the American Institute of Mining, Metallurgical & Petroleum Engineers, New York, N.Y., Feb. 16, 1975.
In the extraction of metals, such as silver and gold from metal-bearing ore by the leaching process, the recovery of metals usually is far inferior to the recovery rates achieved by other known methods, as for instance those including roasting and flotation. The reason for this inferior recovery is the inability efficiently to heap leach rock which has been finely ground, because the leaching solution cannot satisfactorily filter through the densely packed ore particles. On the other hand, leaching ore crushed to a size common in heap leaching, for instance two to five centimeters, results in poor, and particularly uneven penetration of the leaching solution into the crushed ore. This inferior penetration is partly the result of the larger size of the crushed ore pieces, but is also due to the stationary position of the crushed ore.
Normally, in heap leaching, the leaching solution will pursue that downward path through the rock pile or heap, which is the path of least resistance, a phenomenon called channeling, resulting in uneven leaching action. It is possible, for the purpose of improving recovery through the rearranging of ores and diverting channeling, to set off small explosive charges within the leach pile. This approach, however, has the drawback of producing fines, i.e. small rock particles which are densely packed and impair the filtering down of the leaching solution.
It therefore is an object of this invention to increase the overall rate of extraction of the desired metals from crushed ore-bearing rock by the non-agitation leaching method.
It is a further object to increase the effectiveness with which the leaching solution acts upon all of the surfaces of the crushed ore on which metals are exposed, resulting in a more effective utilization of and/or reduction in the amount of leaching solution used and in the total time required to achieve optimum extraction.
It also is an object of the invention to increase the recovery of metals from ores by the heap leaching process by assisting and increasing the penetration of the leaching solution into the interior of the crushed rock by capillary action and dissolving the metals contained therein.
It is a further object of this invention to minimize the adverse effects of channeling, thereby significantly improving the efficacy of the leaching process.
Although the particulars of the method described herein relate, as an illustration, primarily to the extraction of precious metals, it should be understood that the principle of the invention may be applied to the leaching of other metal bearing ores.
In low grade gold and silver bearing ores, the ratio of these metals may vary greatly. Silver may occur at very low levels which sometimes are measured in parts per million, or it may occur at a level a thousand times that of the gold present. Despite a great deal of scientific testing of various ores, leaching generally requires an empirical determination of optimum process conditions, not only for precious but other metals, such as copper and zinc. The operating data enumerated herein therefore are intended to be only illustrative approximations having strict applicability to some, but not necessarily to all metal bearing ores.
In crushing ores for percolation leaching, the empirically determined optimum size of the crushed ore normally represents a comprise between small particle size, which would result in greater extraction as a result of the larger surface area, and the need to minimize fines which would inhibit percolation. Because leach liquor penetrates into ore by capillary action along induced or natural planes, it is important to maximize the surface area of crushed rock exposed to the leach liquor without crushing the rock to a very fine size, an object aided by the present invention.
Heap leaching of low grade silver and gold ores with dilute alkaline cyanide solution is a relatively low capital, low operating cost method. In commercial applications, this kind of heap leaching often is conducted on ore which has been stacked into heaps or piles on watertight drainage pads which permit recirculation and prevent loss of leach liquor. The sides of the ore piles or heaps are sloped and the leach liquor containing the dissolved gold and silver values flows into channels at or below ground level for recirculation. The impervious pads may consist of asphalt, reinforced conrete or other suitable material.
Heap leaching as practiced with respect to the present invention, is percolation leaching in which the cyanide leach liquor is introduced by spraying from perforated plastic pipes or by sprinkling from plastic sprinklers. The volume of leach liquor filtering down through the pile containing from 1,000 tons to upward of 10,000 tons may be between 500 and 2,500 liters per square meter of pile surface area, depending upon the height of the pile. The piles or heaps often are shaped to resemble truncated pyramids ranging in height from two to five meters or more. The height is determined by the size and nature of the crushed ore, the permeability of the pile, the composition, concentration and rate of required replenishment of the leach liquor. The leach liquor is kept alkaline to prevent the loss of cyanide by hydrolysis and the contamination of local water resources. The leaching solution, the optimum concentration of which also must be empirically determined, may contain from two to five grams of sodium cyanide per 1,000 kilograms of solution and the pH is maintained at 10 to 11 with the addition of lime or caustic soda. The leaching time for low grade silver and gold ores crushed to 1 to 5 centimeters may vary from 5 to 45 days. Dissolved silver and gold values can be recovered from pregnant leach liquor by absorption on activated carbon or on zinc, which can then be stripped with alkaline cyanide solution at approximately 100.degree. to 110.degree. centigrade under slight pressure. Other forms of desorption or precipitation may be used and the stripped metal values can be recovered by any methods practiced by those skilled in the art, such as electrowinning.